ReadyNAS products have also been using it for even longer. They don't use the RAID support from BTRFS (they do it themselves in software a level up using mdadm

ReadyNAS products have also been using it for even longer. They don't use the RAID support from BTRFS (they do it themselves in software a level up using mdadm), only the snapshot, COW and general storage. There very occasionally (based on forum posts) seem to be issues with metadata fragmentation for some people when there's a lot of metadata, but otherwise seems very reliable (I've got two myself).